Barack Obama vowed that the United States was "ready to lead once more" as he gave his inaugural address after being sworn in as the 44th President of the United States.

As many as a million people, bundled up against the cold, turned out to watch as Mr Obama swore to "preserve, protect and defend" the US constitution as he become the first black man to hold America's highest elected office.

Addressing listeners around the world, the new president said: "As for our common defence, we reject as false the choice between our safety and our ideals.

"Our Founding Fathers, faced with perils we can scarcely imagine, drafted a charter to assure the rule of law and the rights of man, a charter expanded by the blood of generations. "Those ideals still light the world, and we will not give them up for expediency's sake.

"And so to all other peoples and governments who are watching today, from the grandest capitals to the small village where my father was born: know that America is a friend of each nation and every man, woman, and child who seeks a future of peace and dignity, and that we are ready to lead once more."

The economic ills facing the United States are "a consequence of greed and irresponsibility," Mr Obama said, giving warning that a nation cannot long prosper "when it favours only the prosperous."

The inauguration took place amid unprecedented security. About 8,000 police were deployed and a total of 32,000 military personnel were on duty or on standby.

The inauguration was witnessed on television by a massive global audience and ushers in a new era of American politics.

Among those gathering early to catch a glimpse of Mr Obama was Elizabeth Brooks, a black Washington resident of 30 years, who cried and said she was thinking about the new president's two daughters Malia and Sasha, and a 1963 attack by white racists in which four young black girls died.

She said: "I am remembering the four little girls that were bombed in the 16th Street Baptist church in Alabama – we have two little girls going into the White House tomorrow in their place."

Another supporter Donald Butler, 20, a University of Washington student, said: "I didn't think I would see a black president in my generation. I just had to be here."